1. Field of the Invention
The present invention relates to the manufacture of a hollow aerofoil, in particular the manufacture of a hollow aerofoil component for use in a gas turbine engine.
Aerofoil shaped components are used throughout gas turbine engines. For example, aerofoil shaped stator vanes and rotor blades are used to guide gas through the engine, for example both in the turbine and the compressor, including the fan and associated guide vanes.
2. Description of the Related Art
Weight reduction is an important consideration in gas turbine engines, particularly, although not exclusively, for gas turbine engines used to power aircraft. Generally, the lower the weight of the component the better the performance of the aircraft to which it is fitted, for example in terms of fuel consumption. To this end, it is known to use hollow aerofoils, e.g. rotor blades and/or stator vanes, in some stages of gas turbine engines.
One method of producing a hollow aerofoil involves forming the structure using a skin. This involves creating an internal cavity (which may be filled with another, lighter weight, material) using hot creep or super plastic forming processes. Such processes may generate aerofoils with some advantageous properties, such as thin skin thickness and tight dimensional tolerance, but they involve significant material wastage. This material wastage makes these processes expensive, due at least to high material cost for a given size of hollow aerofoil component.
An alternative method or producing hollow aerofoil components involves attaching a plate to an aerofoil structure out of which a pocket has been machined. The plate is placed into the pocket and attached (for example welded or bonded) therein to produce a hollow aerofoil component.
An advantage of producing the hollow aerofoil by using a plate to cover a pocket in an aerofoil structure is that there is less material wastage than using a skin to produce the hollow aerofoil. However, the dimensional tolerances are not so accurate. This may be because distortion is introduced in the process of attaching the plate to the pocketed aerofoil. Additionally, tolerance errors may “stack-up” in the process used to produce the pocketed aerofoil, the process used to produce the plate, and the process/feature used to locate the plate into position in the pocket, which typically involve placing the plate onto a supporting ledge inside the pocket.
The lack of dimensional accuracy means that the plate generally has to be manufactured to be thicker than would otherwise be required. For example, the extra thickness may be required in order to ensure that there is enough material to be machined back to produce the desired aerofoil shape after it has been fixed into the pocket. Without the extra thickness, the dimensional variation resulting from tolerance “stack-up” and/or distortion may mean that there is not sufficient material to produce the desired aerofoil shape in some of the aerofoils produced by the method.
However, this extra thickness means both that the component is heavier than it would otherwise need to be, and also that there is more material wastage.
It is therefore desirable to manufacture hollow aerofoil components by using a plate to cover a pocket (so as to take advantage of the generally lower material wastage), but with improved dimensional tolerance.